Electric Boat Propulsive Power Explained.

Why kW to hp conversion formulas aren't that helpful in electric boats

One of the first questions people ask us is what horsepower does a kW engine equate to?

And often they have heard of the international hp to kW conversion formula. Which is 1kW = 1.34102209 hp

This is fine, in theory.

However, in practice, we find that calculation is not at all useful.


Because it doesn't take into account the instant torque. And the advantages of electric in the low to mid-range RPMs. The RPMs you actually need to get on the plane or get going - or dock for that matter.

So that's why we refer to propulsive power at the propeller as the important power equation. And why we comfortably say that whatever propellor the Ć«lectric outboard can drive, that's the hp equivalent.

So let us explain how this works.

In a combustion engine, the engine has to develop enough RPM (revs per minute) to reach idle speed. This usually occurs at around 1000 RPM, and you are in idle until that point.

Now you can get into gear and start to develop the advertised hp. But the fully advertised hp doesn't occur until the engine has reached around 5/6000 RPMs. So if you are at mid-range, you have not got all the hp of the engine available to you. An example would be a 40 hp outboard, which would only have around 25hp or less available at the low to mid-range of its RPMs.

Now electric outboards behave very differently. They have full power (instant torque) available from the first turn of the propeller. And a flat torque curve.

This power is important in planing boats, as you want to efficiently get out of the displacement hole and on the plane. Then you can reduce power (and amp draw).

In displacement boats, this instant torque gives you high torque at the low range RPMs - perfect for creating that momentum you need to get the boat traveling. The ideal for displacement boat efficiency is to use the biggest propeller you can, turning at the slowest rate.

So back to propulsive power.

When you are looking at electric outboards, we will ask you what propeller your current outboard turns (pitch and diameter). From there we can work out which is the right kW outboard for your boat. By the way all our outboards are made in Australia.

For further information

We've just released Australia's first 30kW fully electric outboard. For those of you wondering about the power of these engines, instant torque and a flat torque curve means it has the propulsion power at the propeller you'd be looking at in a 40HP internal combustion engine (ICE). Operating cost savings over 10 years are pretty impressive as well. Contact us for a comparison sheet of costs of running 30kW electric vs petrol.

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Contact us for a comparison sheet

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